An implantable medical device is implanted in a patient to monitor, among other things, electrical activity of a heart and to deliver appropriate electrical therapy, as required. Implantable medical devices (“IMDs”) include, for example, pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (“ICD”), and the like. The electrical therapy produced by an IMD may include, for example, the application of stimulation pulses including pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to its normal sinus rhythm.
When an IMD is implanted, one or more leads are located within or proximate to the heart. In many applications, active fixation cardiac leads are implanted. The leads may represent bradycardia or tachycardia leads. During the implantation process, the physician seeks to consistently implant leads at a desired position and with a desired degree of fixation within the cardiac tissue in order to maintain long term stability. A high level of skill and experience is generally required by the physician in order to consistently implant leads for long term stability. During implantation, the physician may be unduly conservative and not sufficiently affix the fixation element of an implantable lead into the myocardial tissue. Insufficient attachment of the fixation element to the myocardial tissue may lead to early dislodgement of the lead. Conversely, the physician may be unduly aggressive and over extend a helix from the lead into the cardiac tissue. Excessive helix extension potentially could lead to certain undesirable consequences such as coring, excessive tissue damage, perforation, micro perforation and the like. Typically, leads provide a very low degree of tactile feedback while the physician is actively attaching a helix to the tissue. This low degree of tactile feedback increases the difficulty for the physician in deciding how much to extend the lead helix.
Heretofore, no objective measure has been available to the physician, during lead implantation that could inform the physician regarding the extent to which a lead fixation element is properly secured to the myocardial tissue. A need remains for a mechanism to provide fixation related information to the physician that will assist the physician to provide more consistent fixation and increase safety to patients during implantation and post IMD implantation.
In accordance with certain embodiments, methods and systems are provided for measuring current of injury (COI) during lead implantation. The methods and systems sense cardiac signals from a lead within a chamber of the heart while the lead is in a pre-fixation position and capture a baseline waveform from the cardiac signals while the lead is in the pre-fixation position. The baseline waveform represents an interface between the lead and a tissue region proximate a tip of the lead before the lead is actively attached to the tissue region of the heart. The methods and systems sense cardiac signals from the lead within the chamber of the heart when the lead is in a post-fixation position and capture a post-fixation waveform from the cardiac signals when the lead is in the post-fixation position. The post-fixation waveform represents an interface between the lead and the tissue region proximate the tip of the lead after the lead is actively attached to the tissue region of the heart. The methods and systems calculate a COI indicator based on an automatic comparison of the baseline and post-fixation waveforms.